This invention relates to active pixel sensors with reduced fixed pattern noise.
Image sensors find applications in a wide variety of fields, including machine vision, robotics, guidance and navigation, automotive applications, and consumer products. Imaging circuits often include a two-dimensional array of photo-sensors each of which forms one picture element (pixel) of the image. Light energy emitted or reflected from an object impinges upon the array of photo-sensors and is converted by the photo-sensors to electrical signals. The individual photo-sensors can be scanned to read out and process the electrical signals.
One class of solid-state image sensors includes active pixel sensors (APS). An APS is a light-sensing device with sensing circuitry inside each pixel. Each active pixel includes a sensing element formed in a semiconductor substrate and capable of converting optical signals into electronic signals. As photons strike the surface of a photoactive region of the solid-state image sensors, free charge carriers are generated and collected. Once collected, the charge carriers, often referred to as a charge packet, are transferred to output circuitry for processing.
In an exemplary APS, charge carriers are collected in the photo-site via a photo-gate. The charge packet is stored in spatially defined depletion regions of the semiconductor, also known as potential wells, in the semiconductor substrate beneath the photo-site. The charge packet then is transferred to an isolated diffusion region via a transfer gate. The diffusion region receives the charge from the photo-gate well and sends a corresponding electrical signal to the pixel amplifier for further processing.
The near-surface potential within the semiconductor can be controlled, for example, by the potential of an electrode or gate near the semiconductor surface. If electrodes are at different voltages, they will form potential wells of different depths. Free positive charges (e.g., holes) move from a region of higher potential to a region of lower potential. Similarly, free negative charges (e.g., electrons) move from the region of lower potential to the region of higher potential.
An imager with a complementary metal-oxide semiconductor (CMOS) active pixel array can be operated in a rolling shutter mode in which each row of the array is exposed at different instants of time. However, for some applications, such as high-speed photography, it is desirable to operate the imager in a frame shutter mode in which all pixels have substantially identical integration stop and start times. Unfortunately, the use of multiple transfer gates in the latter type of imager can result in an undesirable fixed pattern noise (FPN). The FPN, which is a function of the threshold voltages of the transfer gates, generally differs from pixel to another. While errors resulting from the FPN can sometimes be ignored at low image gains, when the image gain is increased, such errors become significant.